參考資訊：
https://www.nandland.com/vhdl/tips/tip-convert-numeric-std-logic-vector-to-integer.html#Numeric-Integer-To-Signed

Conversion	Method
Integer to Signed	to_signed(src, dst'length);
Integer to Std_Logic_Vector	std_logic_vector(to_unsigned(src, dst'length));
Integer to Unsigned	to_unsigned(src, dst'length);
Std_Logic_Vector to Integer	to_integer(unsigned(src));
Std_Logic_Vector to Signed	signed(src);
Std_Logic_Vector to Unsigned	unsigned(src);
Signed to Integer	to_integer(src);
Signed to Std_Logic_Vector	std_logic_vector(src);
Signed to Unsigned	unsigned(src);
Unsigned to Integer	to_integer(src);
Unsigned to Signed	signed(src);
Unsigned to Std_Logic_Vector	std_logic_vector(src);